1. Field of the Invention
This invention relates to the packaging of biological materials, especially fresh produce.
2. Introduction to the Invention
Fruit and vegetables, and other respiring biological materials, consume oxygen (O.sub.2) and produce carbon dioxide (CO.sub.2) at rates which depend upon temperature and upon the particular material and the stage of its development. Their storage stability depends on the relative and absolute concentrations of O.sub.2 and CO.sub.2 in the atmosphere surrounding them, and on temperature. Ideally, a respiring material should be stored in a container having a total permeability to O.sub.2 and a total permeability to CO.sub.2 which are correlated with (i) the atmosphere outside the package (usually air), (ii) the rates at which the material consumes O.sub.2 and produces CO.sub.2, and (iii) the temperature, to produce an atmosphere within the container (the "packaging atmosphere") having the desired O.sub.2 and CO.sub.2 concentrations for preservation of the material. The total permeability to water vapor may also be significant. This is the principle behind the technology of controlled atmosphere packaging (CAP) and modified atmosphere packaging (MAP), as discussed, for example, in U.S. Pat. Nos. 4,734,324 (Hill), 4,830,863 (Jones), 4,842,875 (Anderson), 4,879,078 (Antoon), 4,910,032 (Antoon), 4,923,703 (Antoon), 5,045,331 (Antoon), 5,160,768 (Antoon) and 5,254,354 (Stewart), copending, commonly assigned U.S. patent application Ser. No. 08/759,602 filed Dec. 5, 1996 (Docket No. 10621.2 US), published as International Publication No. WO 96/38495 (Application No. PCT/US96/07939), and European Patent Applications Nos. 0,351,115 and 0,351,116 (Courtaulds). The disclosure of each of these documents is incorporated herein by reference.
The O.sub.2 transmission rate (referred to herein as OTR) and CO.sub.2 transmission rate (referred to herein as COTR), of a body composed of a particular material, are the amounts of O.sub.2 and CO.sub.2, respectively, which will pass through a defined area of that body under defined conditions. The total permeabilities of a container to O.sub.2 and CO.sub.2 depend, therefore, upon the areas, OTRs and COTRs of the various parts of the container.
The preferred packaging atmosphere depends on the stored material. For many materials, the preferred concentration of O.sub.2 is less than the preferred concentration of CO.sub.2. For example, broccoli is generally best stored in an atmosphere containing 1-2% O.sub.2 and 5-10% CO.sub.2 ; berries are generally best stored in an atmosphere containing 5-10% O.sub.2 and 10-20% CO.sub.2 ; and cherries are generally best stored in an atmosphere containing 5-8% O.sub.2 and 10-20% CO.sub.2. In order to produce a packaging atmosphere having a high ratio of CO.sub.2 to O.sub.2, the container should have a low ratio of total CO.sub.2 permeability to total O.sub.2 permeability. The term R ratio is used herein to denote the ratio of COTR to OTR for a particular material or the ratio of total CO.sub.2 permeability to total O.sub.2 permeability of a container or part of a container.
Respiring biological materials are normally stored at temperatures substantially below normal room temperature, but are often exposed to higher temperatures before being used. At such higher temperatures, the respiration rate increases, and in order to maintain the desired packaging atmosphere, the permeability of the container preferably increases sharply between storage temperatures and room temperature.
Respiring biological materials are generally stored in sealed polymeric containers. Conventional polymeric films, when used on their own, do not provide satisfactory packaging atmospheres because their OTR and COTR values are very low and their R ratios are high. Microporous polymeric films, when used on their own, are also unsatisfactory, but for different reasons; namely because their OTR and COTR values are very high and their R ratios close to 1.0. It has been proposed, therefore, to make use of containers which comprise
(i) one or more barrier sections which are relatively large in area and are composed of materials having relatively low OTR and COTR values (e.g. are composed of a conventional polymeric film), and PA1 (ii) one or more atmosphere-control members which are relatively small in area and are composed of a microporous film, and which provide at least a large proportion of the desired permeability for the whole container. PA1 (a) comprises PA1 (b) can be sealed around a respiring biological material to provide a sealed package which is surrounded by air and which contains a packaging atmosphere around the biological material; PA1 (a) a gas-permeable membrane; and PA1 (b) an apertured cover member which, when the container has been sealed around a respiring biological material to provide a said sealed package, lies between the gas-permeable membrane and the air surrounding the package; PA1 the gas-permeable membrane having, in the absence of the apertured cover member, PA1 the apertured cover member being composed of PA1 (a) a sealed container, and PA1 (b) within the sealed container, a respiring biological material and a packaging atmosphere around the biological material; PA1 (a) a gas-permeable membrane; and PA1 (b) an apertured cover member; PA1 the gas-permeable membrane having, in the absence of the apertured cover member PA1 the apertured cover member being composed of
However, for containers of conventional size, the preferred total O.sub.2 permeability, although larger than can be provided by the barrier sections alone, is still so small that the control members need to be very small in area. Such very small control members are difficult to incorporate into containers, and can easily become blocked in use. In addition, the OTR of microporous films does not change much with temperature.
As described in copending commonly assigned application Ser. No. 08/759,602 and corresponding International Publication No. WO 96/38495 (referenced above), much improved results can be obtained through the use of atmosphere-control members composed of a membrane prepared by coating a thin layer of a polymer onto a microporous film. The OTR of these membranes is such that the atmosphere-control members are of practical size. Furthermore, through appropriate choice of the coating polymer, the membranes can have OTRs which increase sharply with temperature. However, although the membranes are very satisfactory for many purposes, they often have R ratios which are higher than is optimal when the desired packaging atmosphere contains a relatively large proportion of CO.sub.2.